1. Field of the Invention
The present invention relates to an AFT (automatic fine tuning) circuit for use in television receivers, videocassette recorders, and similar appliances.
2. Description of the Prior Art
AFT circuits are used to control the local oscillation frequency of a tuner so that a correct video intermediate frequency carrier (58.75 MHz in Japan) is obtained. They are also used to achieve tuning (automatic channel search and channel presetting) in cooperation with a horizontal synchronization detection signal or other.
Conventionally, automatic fine tuning is achieved by the use of an AFT signal generated by a frequency discriminator provided exclusively for this purpose. In recent years, automatic frequency tuning has come to be achieved more and more by the use of an AFT voltage that is obtained as a result of direct F/V (frequency-to-voltage) conversion of the output of a VCO (voltage-controlled oscillator) responsible for the generation of the video intermediate frequency carrier.
FIG. 8 shows an example of a conventional AFT circuit of the more recent type. In FIG. 8, numeral 1 represents a tuner consisting of an RF circuit 3 for amplifying an RF (radio frequency) signal received via an input terminal 2, a mixer 4, and a local oscillator 5. The output of the tuner 1 is transmitted through a SAW (surface acoustic wave) filter 6 to a video processing circuit.
The video processing circuit consists of a PLL (phase-locked loop) circuit 7 for generating a video carrier, and a video detector 8. The PLL circuit 7 includes a VCO 11, a phase comparator 9 for comparing the phases between the output of the VCO 11 and the VIF (video intermediate frequency) carrier of the received signal, and a low-pass filter 10 for smoothing the comparison output from the phase comparator 9. The video detector 8 not only detects a video signal, but also separates an intercarrier audio signal having a 4.5 MHz carrier.
The output of the VCO 11 is converted by a F/V converter 12 into a voltage, and this voltage is supplied as an AFT voltage to a channel selector 13 provided with a tuning microcomputer or frequency synthesizer. The channel selector 13 also receives an output from a horizontal synchronization detection circuit (not shown) via a separate route.
The AFT voltage varies with the VIF frequency as shown in FIG. 9. In FIG. 9, the point B indicates the frequency P.sub.1 (58.75 MHz) at which the VIF frequency should ideally be locked. In reality, however, it is locked within the range between the point A and the point C. E1 and E2 represent threshold voltages for the channel selector 13. When the AFT voltage is below the threshold voltage E2, the channel selector 13 controls the local oscillator 5 so that the AFT voltage (hence the oscillation frequency of the local oscillator 5) is varied in smaller increments or decrements than usual.
The high level of the AFT voltage is set to be equal to the source voltage V.sub.CC, and the point B is set to be at the voltage of V.sub.CC /2. In FIG. 9, the range W where the AFT voltage is at its low level, i.e. below E2, corresponds to the capture range of the PLL.
When the VIF frequency approaches the ideal frequency from below the point A, it first increases over the point C into the capture range W, and then returns toward the point B, since the slope between the points A and C is steep. Similarly, when the VIF frequency approaches the ideal frequency from above the point G, it first decreases below the point D into the capture range W, and then moves toward the point B.
However, this conventional method, which uses an AFT voltage obtained through F/V conversion of the output of the VCO 11 of the PLL circuit, is defective, because it sometimes allows the VIF frequency to be locked erroneously at the frequency of the audio signal.
For example, assume that, as a result of a tuning operation, the VIF frequency is now approaching the ideal frequency from a frequency higher than it (that is, the frequency of the local oscillation signal is now decreasing). As shown in FIG. 4, the SAW filter 6 is designed to exhibit approximately the same gain (level) at two frequencies P.sub.1 ' and S.sub.1 ', which are respectively 500 kHz higher than the frequencies of the video carrier P.sub.1 (58.75 MHz) and the audio carrier S.sub.1 (54.25 MHz).
Moreover, the SAW filter 6 exhibits a steep drop in its gain for frequencies higher than P.sub.1 ', and exhibits a rise in its gain for frequencies higher than S.sub.1 '. Accordingly, when the local oscillation frequency f.sub.L, decreasing from a higher frequency, comes close to the frequency .gamma. in FIG. 7, the SAW filter 6 exhibits a high gain for the frequency f.sub.L -f.sub.S, whereas it exhibits only a slight gain for the frequency f.sub.L -f.sub.P. In FIG. 7, f.sub.P represents the frequency of the video carrier included in the RF signal, and f.sub.S represents the frequency of the audio carrier included in the RF signal; .alpha. represents the frequency at which f.sub.L is to be tuned, and .beta. represents the frequency at which f.sub.L causes the carriers to have the frequencies P.sub.1 ' and S.sub.1 '.
As a result, the phase comparator 9 receives a signal having the frequency f.sub.L -f.sub.S, compares this signal with the output (having a frequency of approximately 58.75 MHz) from the VCO 11, and causes the VCO 11 to be controlled in accordance with such comparison. Thus, the VCO 11 is locked at the frequency f.sub.L -f.sub.S as soon as this frequency comes within the pulling range (.+-.1.5 MHz around the center frequency) of the VCO 11. Obviously, this is an unwanted result.
Even when the VIF signal is tuned from a frequency lower than the ideal frequency, there is a possibility that the capture range W, if it is not wide enough, is skipped and thus a similar unwanted result is caused. It is possible to avoid this problem by varying the frequency in even smaller increments and decrements within the capture range W. However, this increases the time required to achieve tuning.